Apparatus for coating webs



Aug' 1; 1967 R. PRZYBaLLA AFPARATUS FOR COATNG WBBS Filed March 18, 1965 zNvENmR RUDOLF PRzYaiLLA ATTORN United States Patent Office 3,333568 Patented Aug., 1, 1967 3,333,568 APPARATUS FOR COATING WEBS Rudolf Przybilla, Gotzeuhain, Germany, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a Corporation of Delaware Filed Mar. 18, 1963, Ser. No. 265,644 4 Claims. (Cl. 118-50) This invention relates to a novel and particularly advantageous process for .applying film-forming liquid preparations, especially photographic emulsions, to filmbases, and to devices for carrying out this process.

Methods in utmost variety are already known in the lart for applying film-forming liquid preparations, especially photographic emulsions, to film bases. Among these, the older methods are (l) dipping and (2) *bead roll or Wash-over coating methods. In the latter method a second roll, disposed parallel to the guide roll at a very small spacing, dips partially into the emulsion vat, so that a bead of casting or coating solution forms between the two rolls and assures uniform application. True, this method yields films of utmost uniformity, but it has the drawback that the thickness of the applied film depends on the viscosity properties of the coating liquid and on the rate of coating. The higher the viscosity and the greater the coating rate the thicker will be the applied film. It was made necessary, however, especially by the advent of multilayer films, to case the thinnest possible layers at high speed, so that the earlier coating methods no longer gave satisfactory results.

Other processes known in the art, and not depending on solution viscosity and coating rate for film thickness, are the so-called slit die and cascade coating methods. Whereas in slit die coating liquid usually emerges from the slit orifice over the width of the material to be coated, in cascade coating the casting liquid flows from a supply vat over a fixed inclined surface, straight or slightly arched, and is carried to the point of application by the material to be coated. In this case film thickness is governed solely by the amount of emerging or overflowing liquid. But to attain uniform 'application over the entire film width it is necessary in these processes that the overfiow surface or the effluent orifice of the slit die, respectively, shall be formed with high precision and shall not offer the slightest irregularity; otherwise such defects would cause serious imperfections in casting, e.g. streaked coatings and the like. For the same reason these surfaces, which must be heated so that the coating liquid will not congeal, must show no thermal stress, which in practice cannot be adequately achieved without difi'iculty.

Now it has been found that all these drawbacks can be avoided if in cascade coating or in Operating on the overflow principle the overflowing coating liquid passes over a roll, and is taken up therefrom by the 4passing film base so that it is applied to the film base. In this way the roll can stand still during transfer of the liquid preparation, and can be rotated stepwise after transfer of liquid preparation.

According to a particularly advantageous embodiment of the process according to this invention, however, overflow of the film-forming liquid preparation proceeds over a roll revolving at a suitably slow speed, the rotation rate of the roll being .preferably so adjusted that it is slower that that of the passing film base and the over-fiowing liquid preparation is completely taken up by the passing film base and is not by any chance partially drained away, as in bead coating where it is desirable to maintain a bead of coating material on the roller at the nip. By this means application is effected not only by overflow phenomena, but also by entrainment of the coating liquid on rolls, so that the precision specified above is not essential to these overflow surfaces.

Thus the process according to this invention makes it possible to combine the advantages of the bead coating or Wash-over method, namely utmost uniformity of the coating over the entire width of the film base along With simplicity and ruggedness of the equipment, with the advantage of permitting feed control as in the cascade and slit die coating methods. If application is to be effected With the roll standing still, then it is well to operate at diminished pressure, at least beyond a specified viscosity and film thickness, so that the coating liquid can penetrate deeply enough into the slit between the roll and the surface to be coated. In Operating with a rotating roll it is not necessary, though sometimes desirable, to lower the pressure, since in this case the casting solution is uniformly applied even at relatively high viscosities.

Another advantage of the process according to this invention is that in this case the point of application can be kept free from impurities by simple means, avoiding contamination from the congealing coating liquid. As is known in the art, it is necessary to separate the coating 'device from the point of application at thick, eg., adhesion or spliced areas, and to restore the original position with utmost exactness after passing the adhesion area. Since the fiow of coating liquid cannot be stopped in doing this, the point of application is soiled by drying coating solution. To obtain a fiawless cast film these impurities must obviously be completely removed, which involves a substantial expenditure of time on the overflow surfaces, which are usually curved and hen-ce difficultly accessible, and hence causes a loss of film base material which should not be underestimated. But if overflow proceeds over a movable roll, then the instant the coating device is returned to Operating position the roll can be advanced in stepwise fashion, so that the surface immediately at the point of application is always completely free from impurities. If overflow proceeds over a rotating roll, then the roll is cleaned by passing it through a solvent `and then through a drying zone, so that here again completely fiawless surfa'ces lare always presented at the point of application, without any additional cost in time and labor.

In accordance with the statements made above, a device for carrying out the present process as disclosed by the invention is c'haracten'zed by the fact that it has as the overflow member fo-r the film-forming liquid preparation a roll which is fixed or can be rotated in arcs or increments, or is continuously rotatable, and past which the film base moves while taking up the film-formng liquid.

For this purpose such a device may advantageously be constructed in such a way that it has a vacuum chamber and/or a cleaning bath covering the bottom of the overfiow roll, together with an adjacent drying zone, below the slit between the overflow roll and the surface of the film to be coated.

Such a device is illustrated in the drawing; it is suitable for carrying out the process according to this invention.

The coating liquid, e.g., an aqueous gelatino-silver halide dispersion, is fed to the coating device at position 1. Air bubbles are first removed by the partition. At position 2 the coating liquid reaches the overflow point of roll 3. It then flows in a thin film to application point 4 below position 2, where it comes in contact with the film base carried by support roller 8 and is applied thereto. For uniform application over the entire width Of the fihn, it is important that all points along position 2 shall lie in an exactly horizontal straight line, which is comparatively easy to achieve when a roll is used. If roll 3 is at rest during application of the film-forming liquid, it is advisable to operate under diminished pressure in chamber 5, beyond a specified viscosity and film thicknes's, in'order to achieve uniforrn application over` a drying zone 7. This prevents coating defects which might occur by adhesion of coating liquid at the back of coat ing position 4. This is particularly important for uninterrupted operation over long periods of time.

In the coating device, the liquid level of the coating solution is approximately even with the tangent 'from the VVertical axis of roll 3. The vat that contains the coating solution is placed close to the roll and part of the surface of the roll can serve as a wall of the vat.

The vat can be provided with a suitable jacket for circulation of a heat exchange fluid to control the temperature of the liquid coating composition. Simila'rly, the roll can be provided with suitable conduits for circulation of a heat-exchange medium.

The roll, vat and related parts can be made of any suitable corrosion-resistant metal, eg., stainless steel, nickel and chromium plated steel, or glass-coated iron or steel. V a

What is claimed is:

1. A device for coating webs which comprises:

(1) a vessel for supplying an overflow film of filmforming liquid composition;

(2) a rotata'ble, gravity overflow roll closely adjacent said vessel and adapted to receive the overflow film; i

and (3) a web support roller vadjacent said vessel, there being a narrow slit between the surface of the overflow roll and said roller through which a Web may pass and pick up a film of liquid composition 'from the overflow roll, the slit being such that; the overflow film of liquid composition is completely taken up by said Web, the axis of said roller being not below'the axis of the gravity overflow roll, said slit being below the uppermost surface of said overflow roll.`

2. A device according to claim 1 having: (4) a cleaning bath below'the roll and adapted to cover its bottom surface. 3. A device according to clairn 1 having: (4) a Cleaning bath below the roll and adapted to cover its bottom surface, and (5) a drying zone adjacent said bath. 4. A device according to claim 3 having: (6) a vacuum'chamber below the slit.

. References Cited UNITED STATES PATENTS 2,086,117 7/1937 Case 118-262 X 2,257,373 9/ 1941 Fansel0w 117,-111 2,681,294 6/1954 Beguin 117-34 2,842,092 7/ 1958 Pornper 117-111 X ALFRED L. LEAVI'IT, Primary Examner.V

E. B. LIPSCOMB, Examiner. 

1. A DEVICE FOR COATING WEBS WHICH COMPRISES: (1) A VESSEL FOR SUPPLYING AN OVERFLOW FILM OF FILMFORMING LIQUID COMPOSITION; (2) A ROTATABLE, GRAVITY OVERFLOW ROLL CLOSELY ADJACENT SAID VESSEL AND ADAPTED TO RECEIVE THE OVERFLOW FILM; AND (3) A WEB SUPPORT ROLLER ADJACENT SAID VESSEL, THERE BEING A NARROW SLIT BETWEEN THE SURFACE OF THE OVERFLOW ROLL AND SAID ROLLER THROUGH WHICH A WEB MAY PASS AND PICK UP A FILM OF LIQUID COMPOSITION FROM THE OVERFLOW ROLL, THE SLIT BEING SUCH THAT THE OVERFLOW FILM OF LIQUID COMPOSITION IS COMPLETELY TAKEN UP BY SAID WEB, THE AXIS OF SAID ROLLER BEING NOT BELOW THE AXIS OF THE GRAVITY OVERFLOW ROLL, SAID SLIT BEING BELOW THE UPPERMOST SURFACE OF SAID OVERFLOW ROLL. 